1. Field of the Invention
The present invention relates to a thermal air flow sensor, and more particularly to a thermal air flow sensor suitable for measuring a flow rate of intake air in internal combustion engines.
2. Description of the Related Art
Heretofore, as thermal air flow sensors which are provided in intake passages of internal combustion engines for automobiles etc. to measure flow rates of intake air, thermal type sensors have been primarily employed because they can detect a mass flow directly. Of those thermal air flow sensors, attention has been recently focused on one that is fabricated on a semiconductor substrate of silicon (Si), for example, by using the semiconductor microprocessing techniques. The reason is that such a sensor is economical because it can be mass-produced relatively easily, has a small size and can be driven with low power. Materials of heat generating resistors and temperature measuring resistors formed on semiconductor substrates are generally metallic materials such as platinum (Pt), gold (Au), copper (Cu), aluminum (Al), chromium (Cr), nickel (Ni) and tungsten (W), for example.
As disclosed in JP-A-8-54269, for example, it is also known to employ polycrystalline silicon (Poly-Si) as materials of heat generating resistors and temperature measuring resistors formed on semiconductor substrates. Using polycrystalline silicon as resistor materials provides such advantages that the existing semiconductor manufacturing process can be employed as it is, that a specific resistance value can be controlled by controlling an impurity density, and that polycrystalline silicon has good adhesion with silicon dioxide (SiO2) and silicon nitride (Si3N4) which serve as protective films.
However, specific resistance of polycrystalline silicon itself is so large that the polycrystalline silicon cannot be in itself used as a heat generating resistor for measuring a flow rate of air. For this reason, as disclosed in the above-cited JP-A-8-54269, specific resistance of polycrystalline silicon is reduced by doping an impurity in the polycrystalline silicon.
Doping an impurity in polycrystalline silicon however reduces not only the specific resistance, but also the resistance temperature coefficient. Since a thermal air flow sensor measures a flow rate of intake air based on an amount of heat taken away by the intake air, a large resistance temperature coefficient is required to increase detection sensitivity. In a conventional thermal air flow sensor using a platinum wire, for example, the platinum wire has a resistance temperature coefficient of 3700 ppm/K. On the other hand, it was found that when polycrystalline silicon was doped with an impurity element to such an extent as causing the impurity density to saturate, its resistance temperature coefficient lowered down to below 1200 ppm/K. Such a lowering of the resistance temperature coefficient raised a problem of reducing the detection sensitivity of a thermal air flow sensor and rendering the sensor not practical in use.
An object of the present invention is to provide a thermal air flow sensor in which polycrystalline silicon is used as a heat generating resistor, and which has improved detection sensitivity.
(1) To achieve the above object, the present invention provides a thermal air flow sensor including a heat generating resistor formed on a substrate, wherein the heat generating resistor is formed of a semiconductor thin film in which polycrystalline silicon is mixed with a silicide compound.
With that feature, specific resistance of the heat generating resistor can be reduced without lowering its resistance temperature coefficient. As a result, the sensor has improved detection sensitivity.
(2) In the above sensor of (1), preferably, a metal forming the silicide compound contains at least one of molybdenum (Mo), tantalum (Ta), tungsten (W) and titanium (Ti).
(3) In the above sensor of (1), preferably, the semiconductor thin film contains an impurity element doped therein.
With that feature, it is possible to further reduce the sensitivity.
(4) In the above sensor of (3), preferably, an impurity is doped in the semiconductor thin film so that the semiconductor thin film has a resistance temperature coefficient of not less than 1200 ppm/K and specific resistance of 1 to 10 xcexa9/xe2x96xa1 in terms of sheet resistance.
With that feature, the resistance temperature coefficient can be increased while reducing the specific resistance, thus resulting in improved detection sensitivity.
(5) In the above sensor of (1), preferably, lead wires are directly connected by wire bonding to leads which are connected to the heat generating resistor and are made of the same material as the heat generating resistor.
With that feature, the number of manufacturing steps can be cut down.